This invention relates to a process for preparing carbon microballoons by starting with a particular pitch in admixture with a low-boiling organic solvent as a foaming agent.
Carbon microballoons are highly valued as lightweight carbon materials having heat resistance, chemical resistance and other advantages which give them wide utility as low-temperature thermal insulators, composite materials with metal or inorganic matter, nuclear reactor materials, electroconductive plastic materials, and the like.
The preparation of carbon microballoons is disclosed in Y. Amagi et al., U.S. Pat. No. 3,786,134 (Japanese Pat. Publication No. 49-30253); Japanese Pat. Publication Nos. 50-29837 and 54-10948.
Y. Amagi et al., U.S. Pat. No. 3,786,134 (Japanese Pat. Publication No. 49-30253) discloses a preparation of pitch microballoons by homogeneously mixing a hard, highly aromatic pitch having a softening point of 60.degree. to 350.degree. C., a 0-25% nitrobenzene-insoluble moiety and a hydrogen/carbbon atomic ratio of 0.2 to 1.0 with a low-boiling organic solvent, dispersing the mixture into water in the presence of a protective colloid, adjusting the content of the solvent in the resulting microspheres to 0.2 to 10%, flash heating said microspheres whereby the solvent therein functions as blowing agent to form hollow pitch microspheres, and rendering the microspheres infusible by heating them with an oxidizing fluid, and baking to carbonization at a temperature of 600.degree. to 2,000.degree. C. in an inert atmosphere. The pitch as specified-above used in Amagi is selected since the use of a pitch having a softening point above 350.degree. C. or a more than 25% nitrobenzene-insoluble moiety does not result in uniform hollow microspheres because such pitch is hardly mixed homogeneoulsly with a low-boiling organic solvent, thus causing a heterogeneous dispersion and uneven foaming. In Amagi, the pitchs which fulfill the desired requirements are those obtained by removing low-molecular weight components by distillation or extraction.
As described above, the pitch used in Amagi is a particular hard, highly aromatic pitch containing free carbon as well as high-molecular weight components such as mesophase pitch unremoved therefrom. When such a pitch is used for the preparation of carbon microballoons, it is impossible to simultaneously render the pitch-solvent mixture particles hollow and spherical in a single step of flash heating. Amagi, therefore, employed two separate steps of dispersing the pitch-solvent mixture particles in hot water to make them spherical, and flash heating the already spherical particles to foam and make them hollow.
More illustratively, the pitch used in Amagi containing the high-molecular weight components unremoved therefrom are incompatible with the low-boiling organic solvent, and are unable to homogeneously mix with the low-boiling organic solvent. Therefore, the pitch-solvent mixture particle prepared from the pitch of Amagi would not foam into a spherical microballoon unless the pitch-solvent mixture particle is rendered spherical by the dispersion prior to the foaming step.
Moreover, a pitch having a high softening point is particularly difficult to homogeneously mix with the low-boiling organic solvent. When such a pitch is used for the preparation of the carbon microballoons, the pitch needs to be admixed with an excess amount of the solvent. After the dispersion into hot water, the content of the solvent should be adjusted prior to the foaming step, since yield of the spherical carbon microballoons would be markedly reduced without such an adjustment of the solvent content.
The method disclosed in Japanese Pat. Publication No. 50-29837 can only produce microballoons of a limited size. The microballoons prepared by such a methods, despite their many improved properties, have not found a wide variety of applications because of the expense due to the complexity of the methods.
The method disclosed in Japanese Pat. Publication No. 54-10948 produces microballoons which are not completely spherical in shape and often have cracks in their shell.
A method for producing solid spherical particles of carbon and of activated carbon is described in H. Kaji et al., U.S. Pat. No. 4,420,443. In this method, a mixture of a pitch and a viscosity-reducing agent is melt extruded into a form of string-like, bodies which is broken into stick-like bodies, and these stick-like bodies are thermally deformed in hot water into spherical particles from which the viscosity-reducing agent is removed to obtain spherical, but solid particles of carbon or activated carbon. This method, however, does not provide hollow carbon microballoons.
A method for producing solid spherical particles of carbon is also disclosed in Z. Shiiki et al., U.S. Pat. No. 4,273,675. In this method, a pitch is mixed with a viscosity-reducing organic solvent, a softening point-elevating agent, and a void formation-preventing agent, and the mixture is then fused and shaped into spherical, but solid particles.
Although these two methods by Kaji and Shiiki disclose the preparation of the solid, spherical carbon microspheres, an additional troublesome step of adjusting the solvent content as disclosed in Amagi would be required to further foam these solid, spherical carbon microspheres into the hollow carbon microballoons.
Consequently, although the above-described prior art methods may disclose independent processes for the preparation of the spherical, solid carbon microspheres and the preparation of the hollow, spherical carbon microballoons, these processes can not be readily combined to realize a single flash heating step wherein spherical hollow carbon microballoons are formed from irregularly shaped pitch-solvent particles.
Inventors of the present invention have found that these processes can simultaneously be carried out in a single flash heating step when the pitch used in the preparation of the carbon microballoons are free of free carbon and high-molecular weight components such as mesophase pitch and spherulites.